This invention relates to nuclear magnetic resonance (NMR) imaging methods and apparatus and more particularly to a method of shimming the magnets used with such apparatus.
In an NMR imaging sequence, a uniform polarizing magnetic field B.sub.O is applied to an imaged object along the z axis of a spatial Cartesian reference frame. The effect of the magnetic field B.sub.O is to align some of the object's nuclear spins along the z axis. In such a field the nuclei resonate at their Larmor frequencies according to the following equation: EQU .omega.=.gamma.B.sub.O ( 1)
where .omega. is the Larmor frequency, and .gamma. is the gyromagnetic ratio which is constant and a property of the particular nucleus. The protons of water, because of their relative abundance in biological tissue are of primary interest in NMR imaging. The value of the gyromagnetic ratio .gamma. for the protons in water is about 4.26 kHz/Gauss. Therefore in a 1.5 Tesla polarizing magnetic field B.sub.O, the resonance or Larmor frequency of protons is approximately 63.9 MHz.
In a two-dimensional imaging sequence, a spatial z axis magnetic field gradient (G.sub.z) is applied at the time of a narrow bandwidth RF pulse such that only the nuclei in a slice through the object in a planar slab orthogonal to the z-axis are excited into resonance. Spatial information is encoded in the resonance of these excited nuclei by applying a phase encoding gradient (G.sub.y) along the y axis and then acquiring a NMR signal in the presence of a magnetic field gradient (G.sub.x) in the x direction.
In a typical two dimensional imaging sequence, the magnitude of the phase encoding gradient pulse G.sub.y is incremented between the acquisitions of each NMR signal to produce a view set of NMR data from which a slice image may be reconstructed. An NMR pulse sequence is described in the article entitled: "Spin Warp NMR Imaging and Applications to Human Whole Body Imaging" by W. A. Edelstein et al., Physics in Medicine and Biology, Vol. 25 pp. 751-756 (1980).
The polarizing magnetic field B.sub.O may be produced by a number of types of magnets including: permanent magnets, resistive electromagnets and superconducting magnets. The latter, superconducting magnets, are particularly desirable because strong magnetic fields may be maintained without expending large amounts of energy. For the purpose of the following discussion, it will be assumed that the magnetic field B.sub.O is maintained within a cylindrical magnet bore tube whose axis is aligned with the z-axis referred to above.
The accuracy of the image formed by NMR imaging techniques is highly dependant of the uniformity of this polarizing magnetic field B.sub.O. Most standard NMR imaging techniques require a field homogeneity better than .+-.4 ppm (.+-.250 Hz) at 1.5 Tesla over the volume of interest, located within the magnet bore.
The homogeneity of the polarizing magnetic field B.sub.O may be improved by shim coils, as are known in the art. Such coils may be axis-symmetric with the z or bore axis, or transverse to the z or bore axis. The axis-symmetric coils are generally wound around a coil form coaxial with the magnet bore tube while the transverse coils are generally disposed in a so-called saddle shape on the surface of a coil form. Each such shim coil may be designed to produce a magnetic field corresponding to one spherical harmonic ("associated Legendre polynomial") of the magnetic field B.sub.O centered at the isocenter of the magnet. In combination, the shim coils of different order spherical harmonics may correct a variety of inhomogeneities. Among the lowest order shim coils are those which produce a linear gradient along one axis of the spatial reference frame.
Correction of the inhomogeneity of the polarizing field B.sub.O involves adjustment of the individual shim coil currents so that the combined fields of the shim coils just balance any variation in the polarizing field B.sub.O to eliminate the inhomogeneity. This procedure is often referred to as shimming.